exportcert command: Export only if total hours contributed is positive and non-zero

[sharan8]

Previously,

• Records with 0 hour values would still be included in the exported certificate.
• Volunteers with all records having 0 hour values (thus no hours contributed at all) would still have their certificates exported.

This is meaningless and doesn't achieve the intended purpose of the certificate (verification and formal record provision).

Thus, the following change was made:

• ExportCertCommand now goes through an additional layer of filtering through records to surface only the ones with a non-zero, positive hour value. Thus the above two instances are accordingly managed.

RecordContainsNonZeroHourPredicate has been created for this additional layer of filtering.

[coveralls]

Coverage increased (+0.01%) to 65.912% when pulling 695efa7 on sharan8:exportcert-command into 36515dc on CS2103-AY1819S1-W16-2:master.